AbstractDuring an effusive-extrusive eruption, the capability of an active lava body (flow or dome) to radiate thermal energy depends on how the lava discharge rate is accommodated by the expansion of the magma body and by the cooling of its surface. This feature can be described by a single empirical parameter, defined "radiant density" (c(rad): in J m(-3)) that, for a given discharge rate, expresses the efficiency of the lava body to change its area and/or to insulate its inner core, thus modulating the heat radiated from the active surface. By comparing the Volcanic Radiative Energy (VRE; in J), detected by MODIS during 28 eruptions at 18 active volcanoes, with their erupted lava volumes (Vol; in m(3)), we show that the radiant density (c(rad)=VRE/Vol) is inversely proportional to the silica content of the erupted lava. Basic lavas (45-52 wt.% SiO2) have the highest observed radiant density (1 to 4 x 10(8) J m-(3)) while intermediate (52-63 wt.% SiO2) and acidic (>63 wt.% SiO2) lavas show a gradually lower radiant densities (1.5 to 9 x 10(7) J M-3 and 0.2 to 1 x 10(7) J m(-3) for intermediate and acidic composition, respectively). We regard this correlation as the result of the control that the rheology of lavas exerts on cooling and spreading processes of related bodies. In particular, we found that for any given compositional group the radiant density is essentially related to a "characteristic thickness" of active lavas, at the rime of a satellite acquisition. We suggest that the radiant density of effusive/extrusive lava bodies can be predicted (+/- 50%) by means of an empirical relationship based on the SiO2 content of the erupted lava. This makes this parameter very useful in observing volcanic activity, especially in remote regions where access may not be possible. By measuring the energy radiated during an eruption and by assuming a lava composition (based on the tectonic setting or magmatic province), we suggest that the radiant density can be used to estimate lava discharge rates and erupted volumes for volcanoes characterised by effusive or extrusive activity. (C) 2012 Elsevier B.V. All rights reserved.